Bump-on-Trace (BOT) structures are used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is low. The BOT structures may achieve the same reliability as the conventional bond structures that are based on metal pads.
BOT structures may sometimes suffer from peeling-off. For example, when a device die is bonded to a package substrate through the BOT structures, since there is a significant mismatch between the coefficient of thermal expansion (CTE) of the device die and the CTE of the package substrate, stress may be generated in the resulting package. The stress is applied on the metal traces in the BOT structures, causing the metal traces to peel off from the adjoining dielectric layer in the package substrate.